How to Access Custom tab/buttons created using Ribbon.xml for Visio 2010 using C#4.0-Collection of common programming errors
That callback functions give you IRibbonControl sender. I use sender.Id as a reference and lookup the other properties in the Ribbon.XML string from the GetCustomUI() function. If you change another control’s label (referenced via getLabel=”myFunction” in the XML), be sure to use IRibbonUI.InvalidateControl to refresh it.